Unless you chuck Taco Bell bags out your window onto the freeway shoulder daily or dump buckets of coolant in your neighbor's bushes, just driving your car alone is probably the single most polluting act you do on a regular basis. That is, unless you eat Taco Bell everyday but that's another kind of pollution. According to the Environmental Protection Agency, cars are the biggest contributor when it comes to ground-level ozone, the No. 1 air pollution problem in much of the United States. Cars also give off pollutants - toxins - that are responsible for about 1,500 cancer cases in the U.S. each year. And then there's also acid rain, global warming and lots of other bad things. But cars with internal combustion engines aren't going anywhere. At least not anytime soon.

There are three bad things we can blame on the internal combustion engine: hydrocarbons, nitrogen oxides and carbon monoxide. And there are three ways this crap can get into the atmosphere: through tailpipe emissions, underhood evaporative emissions and refueling emissions. Since the 1970s, automakers have fitted cars with various components designed to lessen the environmental impacts caused by vehicle emissions. Every few years or so, these components become more complex and in turn do a better job. If you live somewhere like California and you're reading this magazine, chances are you're eerily familiar with the state's stringent emissions laws. Most enthusiasts would prefer getting their wisdom teeth pulled than to visit their local Test-Only center for their biennial smog checkup. If you live in some Midwest state where you don't even have to get a smog check, wipe that grin off your face because we're not laughing. But things don't have to be this way. The number of emissions-legal aftermarket components is growing, as is the number of aftermarket parts that remain compatible with OEM-issued emissions components. And that's a good thing. Ripping off emissions components in the name of high-performance isn't just bad for the environment, it can also cause today's modern day engines to actually run worse. Here's a rundown of a typical vehicle's emissions components, what they do, and why you need to leave them alone.

CATALYTIC CONVERTER:The catalytic converter is probably the most notorious of all auto emissions reducing components. The cat is integrated into the vehicle's exhaust stream, just after the manifold and before the muffler. Catalytic converters work best when hot, that's why many OEMs have integrated cats into exhaust manifolds, as close to the combustion process as possible. Most cars on the road today are equipped with three-way cats. That means they help chemically reduce three things: hydrocarbons, nitrogen oxides and carbon monoxide. There are two stages to the modern-day, honeycomb-type or pellet-type cats. The reduction stage is the first and it uses platinum and rhodium coated honeycomb or pellets to reduce nitrogen oxides. As exhaust gases pass through, nitrogen oxides contact the metal-coated ceramic structure and oxygen atoms are separated from nitrogen atoms allowing the formation of harmless oxygen and nitrogen. The oxidation stage makes use of the oxygen just produced to burn the hydrocarbons and carbon monoxide over a palladium and platinum catalyst leaving nothing more than carbon dioxide and water.